The present invention relates to the electronic instrumentation field applied to industrial processes, specifically those related with dehydration of light, heavy and extraheavy crude oils.
One of the variables to be controlled in the dehydration of crudes, is the interface level between the petroleum and water, which is defined as the limit in which these two liquids join together due to the difference in density. In general, this level is controlled by instruments of the pneumatic and electronic type. These devices efficiently control the interface level in the dehydration process of light crudes but not in that of the heavy and extraheavy ones, due to certain characteristic factors such as: temperature, emulsion degree, solids contents and the very small difference in the specific gravity between the crude and water; which adversely influence the performance of the control devices. Among the instruments of the pneumatic and electronic types which are commonly used to control this level, we can mention the following: Pressure differential, Floaters, ultrasonic, capacitive or with thermal resistivity devices. However, as stated before, in the dehydration of heavy and extraheavy crudes there are certain factors which adversely influence performance of the interface level controllers used up to this moment, such as:
1. The temperature is a very important factor in the dehydration process, since, in order to achieve a good separation of the crude and the water, it is required to apply a certain amount of heat. When there are problems in maintaining this temperature, whether due to failure in the temperature controllers or to deficiencies in the heating equipment, the crude cools and the sensors of the interface level controllers are saturated with the crude, and this makes good performance of the controllers very difficult. PA1 2. A great part of the interface level controllers are adapted to operate under ideal conditions (crude-water). However, in almost all dehydration processes for crudes a so called emulsion factor is present which may be defined as the mixture of two immiscible liquids which do not separate under normal conditions, and which is formed by stirring the two liquids. In oil installations this factor is produced by the mixture of crude and water as these fluids are pumped from underground through the pump and the surface equipment. Once this factor is present, there are failures in the performance and thus large amounts of emulsion crude drain towards the rubbish wells from where recovery is difficult. PA1 3. Another factor which affects the performance of the interface level controllers is the presence of solids which are dragged with the crude which is pumped from underground. Certain interface level controllers have sensors with very limited areas and they fail due to the obstruction of the sensor surfaces by the solids and sediments. PA1 4. The small difference in the specific gravity between the heavy crude produced by the deposits and the water associated with same (which contains certain amount of salt) eliminates the possibility of controlling the interface level in controllers of the hydrostatic pressure and floating differential types.
An interface level controlling system of the pneumatic or electronic type, comprises, generally, three basic elements: the sensor, the controller and the final controlling element. The sensor is the element in contact with the two liquids and it transmits a signal or information to the controller in accordance with the changes in the interface level to be controlled. The controller receives the signal or information sent by the sensors, and makes a comparison with a previously selected value (adjustment point) and transmits a command signal to the final control element. This element, in turn, performs an action which permits the maintenance of the interface level at the site to be controlled.
We must consider as a specific example, the control operation of the interface level in a water, crude and gas separator such as those used in the sites of recollection of petroleum. The mixture of crude, water and gas from the deposit enters the separator where the separating process takes place. The gas is released through the upper part, the water which is heavier deposits in the bottom and the petroleum exits through the medium part. While the process takes place, the water is drained through an automatic valve operated by the controller, maintaining in this way the interface level in the site required by the operation.